The present disclosure generally relates to apparatus and methods for treatment of spinal disorders using an intervertebral prosthesis which is disposed in an intervertebral space (or cavity) following removal of a damaged or diseased intervertebral disc. However the varied orthopedic embodiments of this apparatus and the methods used therein constitute the basic concept of the invention of fusion cages implanted throughout the human skeleton.
The bones and connective tissue of an adult human spinal column consist of more than thirty three discrete bones coupled sequentially to one another by a tri-joint complex. Each tri-joint complex includes an anterior disc and two posterior facet joints. The anterior space between adjacent bones are cushioned by collagen spacers referred to as intervertebral discs. The spine nomenclature of these bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine includes the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects to the pelvis.
The spinal column is highly complex in that it includes all these bones and viscoelastic structures coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these conditions, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the factors that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determines the appropriate surgical protocol and implantation assembly. The spine surgical community has accepted intervertebral devices (commonly known as interbody spacers, and allograft transplants) as part of the state of the art and routinely employ such devices in the reconstruction of collapsed inter-vertebral disc spaces.
Surgeons insert these intervertebral devices to adjunctively facilitate bone fusion in between and into the contiguous involved vertebrae. This fusion creates a new solid bone mass, which acts to hold the spinal segment at an appropriate biomechanically restored height as well as to stop motion in a segment of the spine in which the patient is experiencing pain. Items surgically placed in these involved interbody regions can thus stimulate interbody bone in-growth such that the operated anterior spinal segments heal into a contiguous bone mass; in other words, a fusion occurs. Further, the surgical community uses such man-made implants or biological options to provide weight bearing support between adjacent vertebral bodies, and thereby correct or alleviate a variety of mechanically related clinical problems. In this regard, surgeons use intervertebral spinal implants/transplants for surgical therapy for degenerative disc disease (DDD), discogenic low back pain, spondylolisthesis, reconstruction following tumor or infection surgery, and other spine related maladies requiring surgical intervention.
In many implant designs, a relatively hard or sturdy implant construct is formed from a selected biocompatible material such as metal, ceramic, plastic, or carbon fiber-reinforced polymer. This implant construct often has a partially open or porous configuration and is coated or partially filled with a selected bone ingrowth-enhancing substance, such as harvested bone graft supplied from the patient, human donor allograft bone transplant material supplied by a tissue bank, genetically cultivated bone growing protein substitutes, and/or other biological/biochemical bone extenders. Such devices, when implanted into the intervertebral space, promote ingrowth of blood supply and grow active and live bone from the adjacent spinal vertebrae to inter-knit with the implant, thereby eventually immobilizing or fusing the adjacent spinal vertebrae. Such implants also commonly include a patterned exterior surface such as a ribbed or serrated surface, or screw thread geometry, to achieve enhanced mechanical locking with the adjacent vertebrae during the bone ingrowth/fusion process.
With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted into the anterior aspect of the spine. Cylindrical intervertebral discal cages generally comprise a tubular metal body having an external surface threading. They are inserted transverse to the axis of the spine, into preformed cylindrical holes at the junction of adjacent vertebral bodies. The cages include holes through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior of the cage to incite or accelerate the growth of the bone into the cage.
Conventional intervertebral discal cages generally comprises a device with a geometry that mimics the shape of the intervertebral disc, made of plastic, carbon fiber, metal, or human tissue, having an upper and lower surface which are designed to interface with well prepared flat vertebral body endplate structures. These cages are designed to interface transversely to the axis of the spine into completely shelled out disc spaces, the geometry of the cage mirroring the hollow intervertebral disc space. The cages include at least one large graft hole in line with the spinal axis through which the superior and inferior endplates may form an osseous column and fuse. Typically, these holes are packed with a variety of graft, graft enhancing, bone generating, or bone substitute like materials.
Additionally, the spine surgery community has generated several commercially available cages with integrated screws that operate as stand-alone support devices (that is, without supplemental support from an additional construct such as an anterior plate and screws, or posteriorly placed transpedicular screws and rods or screws placed through the facet joints) interbody fusion devices. These devices include the Stalif™, SynFix™, and the VerteBridge™. The Stalif™ is a device for the fusion of the lumbar spine. The implant is inserted and fixed via converging screws passing through pre-drilled apertures of the device that penetrate into the vertebral bodies. The screws are manually placed into the apertures of the device and are driven using an appropriate tool, such as a surgical screw driver. The Stalif™ is available from Centinel Spine, www.centinelspine.com. The SynFix™ is also a device that is placed in an intervertebral space and fixed via diverging screws passing through the device and into the vertebral bodies. Again, the screws are manually placed into the apertures of the device and are driven using a surgical screw driver. The SynFix™ is available from Synthes, Inc., 1302 Wrights Lane East, West Chester, Pa. 19380 (www.synthes.com). The VerteBridge™ is a device for the fusion of the spine in which anchoring blades are press-driven (using a specialized tool) through apertures in the device and into the respective vertebral bodies to fix the device in place. The VerteBridge™ is available through the LDR Spine (www.ldrholding.com).
All of the above-described devices have an anchor which is secondarily added to the initial device. The Stalif™ and SynFix™ devices employ screws while the VerteBridge™ utilizes a blade anchor. Both the Stalif™ and SynFix™ devices require the screws to be inserted at trajectories that are difficult to achieve given common human anatomical structures, especially at the spinal disc space levels of L4-L5-S1. Additionally, the proximal end of the screws may protrude anteriorly, causing potential irritation and erosion to the great and small vessels, and possibly through innocent misadventure snag ureters and surrounding soft tissue as the screw is blindly approximated and then secured into its home/locked position.
The VerteBridge™ has a pair of blades inserted after the initial device is put in place. The blades are supposed to flex enough to curve within the device, and to exhibit sufficient strength to cut through bone. These blades, although flexible, need to be able to hold the vertebral bodies in place in all planes of motion under normal physiologic and, to a degree, superphysiologic conditions. In practice, these features may not always be achieved.
A number of devices have been developed, which employ self-contained anchoring elements that are deployed after the device is placed into the intervertebral space. For example, U.S. Patent Application Pub. No. 2006/0241621 (incorporated herein in its entirety) discloses a device for joining intervertebral members together using a self-drilling screw apparatus. The screw apparatus includes a shell and first and second screw members having tapered ends and threaded bodies that are disposed within the shell. A drive mechanism rotationally drives the first and second screw members from the shell in precisely co-axial, opposite directions, which causes the screw members to embed themselves in the vertebral bodies. U.S. Pat. No. 5,800,550 (incorporated herein in its entirety) discloses a device for joining intervertebral members together using a self-deploying pair of posts. The apparatus includes a body and first and second post members that are disposed within the body. A drive mechanism press-drives the first and second posts from the body in precisely co-axial, opposite directions (longitudinally aligned with the spine), which causes the posts to embed themselves in the vertebral bodies. The problems with these devices include that the co-axial, opposite deployment of the screws/posts is not an ideal configuration for fixing an intervertebral device. Indeed, such a deployment may permit slippage of the device during or after deployment because of the natural stresses applied to the device from the patient's anatomical spinal structures.
Another approach is disclosed in U.S. 2010/0161057, the entire disclosure of which is incorporated herein in its entirety. That publication discloses an intervertebral prosthesis that includes a body having one or more apertures extending transversely therefrom with respect to the longitudinal axis. Respective anchoring elements disposed within the apertures are threaded and deploy in response to a driving rotational force on a gear. The gear is disposed adjacent to, and in meshed threaded communication with, the threaded shaft of the anchoring elements such that rotation of the gear causes rotational torque of the anchoring elements. The driving rotational force on the gear causes the anchoring elements to rotate, deploy from the body, and thread into the vertebral bone of the patient's spine.
Despite the advancements in the art, there is nevertheless a need for a new intervertebral device that includes self-contained anchoring members that deploy in response to rotational, pulling, or pushing driving forces.